It is frequently desirable to generate radio-frequency electronic signals for various signaling, sensing and communications purposes. When radio-frequency (RF) electromagnetic signals must be transmitted, it is often necessary to transmit high power so as to easily overcome distance-related and other losses.
In the past, the term “radio frequencies” was interpreted to mean a limited range of frequencies, such as, for example, the range extending from about 20 KHz to 2 MHz. Those skilled in the art know that “radio” frequencies as now understood extends over the entire frequency spectrum, including those frequencies in the “microwave” and “millimeter-wave” regions, and up to light-wave frequencies. Many of these frequencies are very important for commercial purposes, as they include the frequencies at which radar systems, global positioning systems, satellite cellular communications and ordinary terrestrial cellphone systems operate.
The prior art includes many methods for producing high power radio-frequency signals. For example, a low-frequency or baseband analog signal generator produces signals which are applied to one or more frequency upconverter fed by one or more intermediate frequency generators, to thereby produce the desired high-frequency signal. This process often undesirably results in the production of spurious signals, which in turn requires additional filtering for reducing the amplitude of the spurious signal without significantly affecting the desired signals. The upconverted signal is separately amplified with a high-power amplifier to produce the desired high-power transmit signal. The power amplifier tends to be highly nonlinear when operated efficiently (that is, near saturation), and consequently tends to produce distortion when used for multicarrier applications. Upconverters suffer from similar nonlinearity effects.
In the transmission of signals that include multiple combined signals, such as a plurality of independent channels, modulation techniques with amplitude modulation or both amplitude and phase modulation such as multibit Quadrature Amplitude Modulation (QAM) waveforms, and signals that contain multiple simultaneous beams in the context of phased array antennas, the distortion of the heavily saturated amplifiers adds unwanted components to the transmitted signals. These unwanted components are known as intermodulation products. These intermodulation products may prevent proper system operation. The intermodulation products may be viewed as a form of error. This error limits the amount of information which can be transmitted through the channel. A prior-art approach to enhancing the spectral purity of the transmitted signals can include the use of a power amplifier capable of more power than that actually transmitted, to thereby tend to operate the amplifier in a more linear region of its characteristic. This is an effective solution, but is inefficient in terms of power consumption, and can also be expensive.
The prior art often addresses the efficiency issue by predistortion of the input signal of an amplifier operated in a nonlinear portion of its characteristic, which is more efficient than the linear portion. Another method uses feedforward amplifier techniques which involve the use of amplifiers for the desired signal in conjunction with amplifiers for the distortion, with directional couplers for summing the distortion component in the amplified desired signal with a signal in antiphase with the distortion component. All of these techniques suffer from narrow bandwidth over which distortion cancellation is effective. Additionally, these schemes may be adversely affected by environmental factors such as temperature, which may cause them to fall out of alignment. Even when operating properly, the achievable distortion suppression is not large.
Another prior art solution for producing high transmitter power is that of a direct digital synthesizer (DDS), usually combined with a bandpass filter device in the form of a tunable filter or a switchable analog filter bank, driving a power amplifier. Such a DDS system typically includes a source of signal parameter information, such as phase change per unit time, amplitude as a function of time, and the like. It also includes an accumulator and a phase-to-amplitude converter, often implemented as a “Sine ROM” combined with a polynomial interpolator. ROM stands for read only memory device and a Sine ROM is programmed to look-up an amplitude word output in response to a phase word input. The use of sine ROMs is described, for example, in U.S. Pat. Nos. 4,584,541 and 4,628,286. Further, the DDS system typically includes a digital-to-analog converter (DAC) for converting the amplitude represented by the digital representation into the desired radio-frequency signal.
The use of digital nonlinear techniques for generation of predistortion signal can be introduced into the DDS system, often through appropriate control of the phase and amplitude of the DDS, in order to generate an RF predistorted signal which compensates for the amplifier distortion, particularly when multiple signals must be simultaneously transmitted.
These systems are complex and tend to be limited in the spectral purity of their output signals. When multispectral signals are desired in a DSS system, only the DAC can be common. The DAC converters are subject to amplitude and time errors in producing the reconstruction of the desired RF signals, thereby distorting the single or multiple carriers. The combined effect of the DDS chain limits its utility for generating signals without loss of information. In practice, the predistortion signals produced by DDS systems are limited both by the quality of the DAC in the DDS and by the effectiveness of the predistortion technique itself. While the digital algorithm provides a higher degree of freedom than does an analog system, the digital prediction algorithm or system never fully matches the amplifier distortion and often gives less improvement than might be desired.
Improved or alternative radio-frequency signal generators are desired.